BANCROFT 
LIBRARY 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


With  the  Author's  Compliments 


Geology  of  Pachuca  and 
El  Oro,  Mexico 

BY 

HORACE  V./WINCHELL, 

*-— — 
Minneapolis,  Minn. 


Paper  of  the  American  Institute  of  Mining 
and  Metallurgical  Engineers,  No.  1074-M, 
issued  with  MINING  AND  METALLURGY, 
June,  1921. 


(New  York  Meeting,  February,  1921) 

NEW  YORK,  N.  Y. 
1921 


TRANSACTIONS    OF    THE    AMERICAN    INSTITUTE    OF    MINING    AND    METALLUR- 
GICAL ENGINEERS  jSUBJECT    TO  REVISION] 

No.  1074-M.     ISSUED  WITH  MINING  ANP  METALLURGY  JUNE,  1921 


iO  I. 


J    I 


Geology  of  Pachuca  and  El  Oro,  Mexico 

By  HORACE  V.  WINCHELL,  Minneapolis,  Minn. 

(New  York  Meeting,  February,  1921) 

AN  EXAMINATION  of  the  Pachuca  and  El  Oro  districts  in  July,  1920, 
although  cursory  and  incomplete,  disclosed  facts  of  more  than  passing 
interest  to  the  student  of  ore  deposits;  and  inasmuch  as  the  literature 
on  these  districts  is  limited  and  some  of  their  features  are  little  under- 
stood, a  brief  account  may  prove  valuable  as  reconnaissance  observations 
to  be  amplified  by  more  thorough  and  competent  observers  in  the  future. 
Where  no  complete  monograph  of  important  mining  camps  has  been 
published  a  partial  presentation  of  the  data  is  often  useful,  as  it  some- 
times leads  to  fuller  and  more  accurate  descriptions,  by  stimulating 
discussion. 

The  following  notes  are  presented  with  the  consent  of  Mr.  A.  F.  Main, 
managing  director  of  the  El  Oro  Mining  and  Railway  Co.,  whose 
fund  of  information  and  wide  acquaintance  with  both  districts  alone 
made  it  possible  to  cover  so  much  ground  in  a  short  time.  I  have  also 
to  acknowledge  gratefully  the  aid  of  Messrs.  C.  A.  Lantz  and  D.  S. 
Calland  of  the  Santa  Gertrudis  and  Real  del  Monte  companies,  respec- 
tively, in  connection  with  permission  to  inspect  geological  features  of 
interest  in  their  mines. 

PACHUCA,  HIDALGO,  MEX. 

This  mining  camp  has  been  described  by  Ezequiel  Ordonez1  and  by 
A.  F.  J.  Bordeaux.2  Reported  to  have  been  discovered  in  1522,  it  has 
produced  silver  .valued  at  between  $250,000,000  and  $375,000,000. 
Subsequent  to  the  discovery  here,  by  B.  de  Medina,  of  the  patio  process, 
in  1557,  its  production  was  increased,  and  now,  using  cyanidation,  the 
output  of  the  camp  is  nearly  one-sixth  of  the  production  of  the  world. 

Topography  and  Climate 

The  Pachuca  district  consists  of  a  series  of  mountains  and  valleys, 
the  minor  features  of  which  extend  in  an  easterly  and  westerly  direction 
and  reach  elevations  of  8000  to  10,000  ft.  (2438  to  3048  m.)  above  the 

1  Mining  District  of  Pachuca,  Mexico.     Trans.  (1902)  32,  224. 

2  Silver  Mines  of  Mexico.     Trans.  (1908)  39,  360. 


Copyright,  1921,  by  the  American  Institute  of  Mining  and  Metallurgical  Engineers,  Inc. 


2  GEOLOGY  OP  PACHUCA  AND  EL  ORO,  MEXICO 

sea.  The  range  as  a  whole  constitutes  a  continental  divide;  its  axis  runs 
northerly  and  southerly  and  separates  the  waters  that  flow  westerly 
into  the  Pacific  Ocean  from  those  flowing  easterly  into  the  Gulf  of  Mexico. 
On  the  north  and  east,  the  surface  falls  rapidly  down  to  the  tierra  caliente 
and  is  cut  by  deep  canyons.  In  this  warm  country,  about  4000  ft. 
(1219.2  m.)  below  the  summit  is  a  canyon  2500  or  3000  ft.  (762  or  914.4m.) 
deeper  that  drains  the  region  on  the  eastern  slopes;  here  are  raised  coffee 
and  such  tropical  fruits  as  bananas.  Dense  forests,  heavy  undergrowth, 
and  much  soil  cover  the  land.  Cloudy  weather  prevails  over  a  large  part 
of  the  year;  the  abundant  rains  do  not  run  quickly  off  but  are  held  in  the 
soil  and  in  the  vegetation  that  conceal  the  underlying  rocks. 

On  the  western  slope  we  are  almost  immediately  in  different  atmos- 
phere and  country;  the  vegetation  is  relatively  sparse  and  the  conditions 
semi-arid.  The  rocks  are  either  bare  or  covered  with  laterite;  they  are 
often  weathered  and  rotted  to  a  considerable  depth  and  the  eye  has  an 
unobstructed  view  over  large  areas.  The  sky  is  .clear,  the  sun  beats 
hotly  down,  and  although  there  are  frequent  showers  in  the  afternoon, 
sharply  limited  to  portions  of  the  area,  the  waters  either  run  off  quickly 
or  are  rapidly  absorbed  by  the  rotted  rocks. 

According  to  the  best  weather  data  obtainable,  there  is  a  marked 
difference  in  the  amount  of  rainfall  on  the  two  slopes,  within  the  few 
miles  occupied  by  the  operating  mines.  At  Pachuca,  the  average  rainfall 
for  the  years  1909  to  1912,  inclusive,  is  given  by  Mr.  Lantz  as  11  in. 
per  annum;  according  to  Mr.  Calland,  the  records  of  the  Real  del  Monte 
Co.  show  at  Loreto,  where  the  Pachuca  mill  is  situated,  a  precipitation 
of  21.22  in.  (53.97  cm.);  at  the  village  of1  Real  del  Monte.  32.76  in. 
(83.2  cm.)  and  at  Velasco,  about  3  mi.  (4.8  km.)  north  of  the  Guerrero 
mill  of  the  Real  del  Monte,  59.33  in.  (150.7  cm.).  These  facts  are 
mentioned  because  they  suggest  a  difference  in  the  chemical  activity  of 
surface  waters,  which  as  will  be  explained  later  seem  to  have  played  an 
important  part  in  the  genesis  of  these  extensive  ore  concentrations. 

Geology 

We  have  to  do  here  with  Tertiary  eruptives  similar  to  those  that 
contain  so  many  of  the  mines  of  Mexico  and  the  southwestern  part  of  the 
United  States.  The  underlying  basement  on  which  these  volcanics 
were  deposited  is  probably  Cretaceous  sediments,  but  none  were  ob- 
served in  the  area  covered  by  these  notes. 

The  oldest  rock  seen  is  the  older  andesite.  This  is  a  thick  flow  of 
inconstant  texture  and  varies  from  a  massive  rock  with  few  phenocrysts 
to  a  highly  porphyritic  mass  in  which  are  many  whitened  feldspars  and 
some  altered  pyroxenes  with  occasional  biotites.  Toward  the  north  and 
west  this  typical  andesite  is  succeeded  by  a  quartz  andesite,  or  dacite, 


HORACE   V.    WINCHELL  3 

much  more  siliceous,  lighter  in  color  and  with  many  quartz  phenocrysts 
ranging  in  size  from  Jli2  to  J6  in.  (0.79  to  3.175  mm.)  in  diameter. 
Whether  the  dacite  is  a  phase  of  the  older  andesite  or  a  separate  flow 
was  not  ascertained.  No  definite  line  of  contact  was  seen.  Both  of 
these  rocks  contain  fragments  of  darker  fine-grained  andesite  and  both 
at  times  exhibit  marked  flow  structure.  They  can  hardly  have  been 
intrusives,  for  there  is  no  remnant  of  any  covering  rock.  Moreover, 
at  times  the  vesicular  or  amygdaloidal  texture  characteristic  of  rocks 
cooled  near  the  surface  is  found. 

The  older  andesite  appears  on  the  surface  over  the  greater  portion 
of  the  area  thus  far  developed  by  mining  operations.  It  can  be  traced 
continuously  from  a  point  more  than  1  mi.  (1.6  km.')  east  of  the  Guerrero 
mill  of  the  Real  del  Monte  Co.  to  a  point  west  of  the  Bordo  shaft,  where  it 
passes  beneath  later  eruptives,  about  12  mi.  Its  width  on  the  surface 
is  much  less,  since  its  northern  margin  is  south  of  the  northern  limit  of 
underground  mining  operations  and  may  be  seen  just  north  of  the  valley 
in  which  are  situated  the  Paraiso  and  Santo  Tomas  shafts.  Its  southern 
limit  of  exposure  was  not  traced. 

Within  this  older  andesite  are  east-west  zones  of  extensive  and 
profound  thermal  alteration  in  which  the  rocks  have  been  altered  over 
areas  probably  several  miles  long  and  often  hundreds  of  feet  wide.  The 
altered  rock  is  softer  and  lighter  in  color  than  the  fresh  andesite  and 
carries  occasional  pyrite  crystals,  with  calcite  films  and  veinlets,  and 
even  quartz  incrustations.  It  is  in  these  altered  zones  in  the  older 
andesite  (and  dacite)  that  the  important  ore  deposits  have  been  found 
and  developed. 

The  succeeding  rock  is  later  andesite.  This  consists  of  flows,  breccias 
and  tuffs,  all  highly  porphyritic  and  generally  less  massive  than  the 
older  andesite.  It  is,  however,  sometimes  thick-bedded  and  dense  and  so 
similar  in  appearance  as  to  be  distinguished  with  difficulty  except  on 
weathered  surfaces.  It  weathers  in  various  colors  from  reddish  brown  to 
bluish  or  purplish  gray  and  covers  large  areas  north  of  the  actively  mined 
district.  In  places,  only  a  thin  shell  of  this  rock  is  left;  elsewhere  shafts 
go  down  through  it  into  the  lower  andesite.  In  the  Girault  tunnel,  the 
contact  is  well  exposed;  near  the  Trompillo  shaft  there  is  a  steep  fault  con- 
tact between  the  two  formations  with  the  Vizcaina  vein  lying  between. 
This  later  andesite  has  not  been  considered  promising  ground  for  explora- 
tion and  has  produced  but  little  ore.  Whether  this  prejudice  is  based 
on  its  actual  poverty  or  not  is  a  question  that  can  be  answered  only  by 
future  and  deeper  development,  where  this  later  formation  attains  great 
thickness.  So  far  as  I  could  learn,  the  only  workings  in  it  are  compara- 
tively shallow  (at  least  in  the  central  portion  of  the  camp)  and  do  not 
attain  depths  where,  even  in  the  older  andesite,  valuable  orebodies  are 
first  encountered. 


4  GEOLOGY  OF  PACHUCA  AND  EL  ORO,  MEXICO 

Still  younger,  and  overlying  the  later  andesite,  is  rhyolite.  This 
rock  is  cream  colored  and  occurs  on  the  surface  in  thin  flows  and  tuffs, 
often  much  weathered  and. cut  by  small  arroyos.  Rhyolite  or  quartz 
porphyry  dikes  of  the  same  material  are  also  seen  on  the  surface  and 
underground  throughout  the  district  from  the  Santa  Gertrudis  north  to 
the  Capula  and  from  the  crest  of  the  main  divide  above  Real  del  Monte 
west  to  and  beyond  the  Bordo.  These  dikes  have  various  strikes  and 
dips  but  the  larger  and  more  continuous  ones  strike  and  dip  parallel  to 
the  main  productive  vein  systems.  On  the  surface  and  underground 
these  rhyolite  flows  and  quartz  porphyry  dikes  are  cut  by  the  vein  fis- 
sures, which  are  sometimes  particularly  well  mineralized  and  contain 
large  stopes  where  they  abut  against  or  rest  upon  the  dikes.  At  Capula 
the  vein,  carrying  ore  in  two  or  more  strands,  is  in  a  quartz  porphyry 
dike  of  great  longitudinal  extent  and  from  50  to  100  ft.  thick  dipping 
southerly  about  50°. 

Cutting  later  andesite,  but  of  undetermined  age  relation  to  the 
rhyolite  is  basalt.  This  rock  is  seen  in  east-west  dikes  from  5  to  40  ft. 
(1.5  to  12.2  m.)  thick  and  a  general  northerly  dip  in  the  country  west  of 
the  Pachuca  mineralized  area.  It  is  not  much  weathered.  It  stands  up 
in  bold  outcrops  and  is  dense  and  tough  and  contains  abundant  olivine 
phenocrysts.  The  later  andesite  near  these  dikes  has  been  weathered 
and  oxidized,  and  the  red  belts  have  sometimes  been  mistaken  for  veins. 

Vein    Systems 

So  far  as  could  be  observed,  all  the  veins  of  the  Pachuca  camp  belong 
to  the  same  general  period  of  fracturing.  Although  some  veins  terminate 
against  or  are  faulted  by  others,  the  evidence  tends  to  show  that  all  of  the 
productive  veins  thus  far  developed  were  formed  and  primarily  mineral- 
ized atone  period,  and  from  the  same  agencies  and  structural  disturbances. 
Nevertheless  there  are  two  systems  of  veins;  one  striking  east  and  west 
and  one  striking  north  and  south.  The  east-west  veins  are  said  to  have 
been  first  discovered  and  worked,  although  the  north-south  veins,  which 
are  rich  and  wonderfully  productive,  have  also  been  worked  for  many 
years.  The  latter  occur  in  the  eastern  part  of  the  camp,  largely  in  the 
property  of  the  Real  del  Monte  Co.  Most  of  the  east-west  veins  have 
numerous  branches  extending  both  northeasterly  and  northwesterly, 
forming  what  has  been  termed  "linked  vein"  structure,  quite  typical  of 
rock  fracturing  on  a  large  scale.  Some  of  t  he  branches  extend  through- 
out the  intervening  space  between  the  stronger  east- west  veins;  others 
apparently  die  out  before  reaching  any  great  distance.  The  full  extent 
of  the  north-south  vein  system  has  not  yet  been  disclosed,  nor  is  it  known 
how  many  such  veins  remain  to  be  discovered.  They  seem  to  terminate 
on  the  south  against  the  great  Vizcaina  fissure,  although  the  Dios  te 


HORACE   V.    WINCHELL  5 

Guie  is  said  to  be  displaced  by  the  Vizcaina  and  to  extend  a  short  dis- 
tance south  of  it,  without  however  containing  much  ore. 

The  principal  veins  of  the  east-west  system  are  the  Santa  Gertrudis, 
Fresnillo,  Analcos,  Vizcaina,  Maravillas,  Santa  Ana,  and  Polo  Norte. 
La  Corteza  and  El  Lobo  veins  strike  northwesterly,  while  the  Veta  del 
Tajo,  Cristobal  Colon  and  Florencia  strike  to  the  northeast. 

So  far  as  developed,  the  north-south  veins,  in  their  order  from  west 
to  east,  are  the  Dios  te  Guie,  San  Sabas,  Purisima,  Santa  Ines,  Santa 
Brigida,  and  Veta  de  la  Reina.  The  stoping  width  of  these  veins  varies 
from  3  to  40  ft.,  and  some  of  the  oreshoots,  such  as  that  in  the  Purisima 
(N-S)  vein  are  400  m.  long.  The  large  east-west  veins  contain  some  of 
the  longest  oreshoots  ever  developed,  reaching  1000  m.  in  the  Santa 
Gertrudis  vein  and  as  much  in  the  Vizcaina.  The  horizontal  dimensions 
of  the  oreshoots  generally  exceed  the  dip  length.  The  bearing  of  this 
fact  on  the  question  of  ore  genesis  will  be  suggested  later.  The  dip  of  the 
east-west  veins  is  generally  to  the  south,  at  angles  of  from  65°  to  80°. 
Local  north  dips  are  not  uncommon.  The  north-south  veins,  dip  both 
easterly  and  westerly;  it  is  said  that  the  former  are  the  more  productive. 

The  veins  occupy  planes  of  fracture  and  zones  of  shearing  and  are 
composed  of  crushed  country  rock  more  or  less  completely  replaced 
by  quartz  and  other  vein  minerals.  The  best  veins,  from  the  standpoint 
of  productivity,  are  in  and  course  through  the  zones  of  greatest  rock 
alteration,  and  yet  there  was  occasionally  observed  a  belt  or  barrier  of 
fresher  looking  rock  just  before  coming  to  a  vein.  Such  a  belt  lies  just 
east  of  the  east-dipping,  north-south  Purisima  vein. 

There  are  evidences  of  two  generations  of  quartz  deposition.  The 
first  period  quartz,  which  replaced  the  broken  andesite  and  often  pre- 
sents a  banded  or  curved  structure  suggestive  of  its  deposition  around 
rock  fragments,  is  white  and  bony  or  ivory-like.  Upon  and  around  this 
white,  quartz  and  in  its  interstices  is  a  darker  later  quartz  carrying  silver 
minerals  with  pyrite  and  a  little  galena  and,  still  rarer,  crystals  of  blende 
and  chalcopyrite.  Where  not  replaced  by  quartz,  the  vein  filling  is 
sericitic  or  kaolinic.  The  quartz  is  usually  broken  into  fragments, 
especially  where  the  veins  are  wide,  but  is  sometimes  solid  and  "frozen" 
to  the  hard  country  rock.  Some  veins  have  good  walls;  some  are  in 
"  bad  ground;"  some  are  accompanied  by  well-defined  planes  of  movement 
on  one  or  both  walls  or  in  the  veins  themselves.  The  best  and  most 
abundant  ore  seems  to  be  in  those  veins  that  are  quite  open  to  the  passage 
of  ground  water.  There  is  not  a  great  flow  of  water,  considering  the 
length  of  underground  development.  As  the  rainfall  at  Pachuca  is  so 
much  less  than  that  at  Real  del  Monte,  it  is  probable  the  eastern  mines 
make  more  water  than  those  in  the  western  camp. 

It  is  a  singular  and  significant  fact  that  on  the  western  slope  oxidation 
extends  to  the  lowest  productive  levels,  about  2000  ft.  (609.6  m.)  from 


6  GEOLOGY   OF   PACHUCA    AND    EL   ORO,    MEXICO 

the  surface.  This  is  not  intended  to  imply  that  no  sulfide  minerals 
remain.  The  sulfide  of  silver,  argentite,  is  the  principal  ore  mineral, 
and  pyrite  is  quite  abundant;  but  the  quartz  is  honeycombed,  rhodonite 
and  rhodochrosite  are  more  or  less  altered  to  psilomelane  and  pyrolusite, 
horn  silver  is  apparent  and  calcite  coatings  are  found  on  the  joints  and 
in  cavities.  The  effect  of  weathering  is  naturally  greater  as  one  ap- 
proaches the  surface,  and  indeed,  not  only  the  silver  but  the  quartz 
itself  seems  to  have  been  dissolved  out  of  the  upper  portion  of  the  veins 
and  carried  downwards.  It  is  only  occasionally  that  quartz  and  ore 
persist  upwards  to  the  grass  roots.  Few  stopes  extend  high  enough  to 
make  their  presence  known  by  surface  settling,  and  the  average  pros- 
pector acquainted  with  the  camp  states  without  hesitation  that  good  ore 
can  hardly  be  expected  short  of  400  or  500  ft.  (121.9  or  152.4  m.)  in 
depth.  This  has  a  familiar  sound,  but  seems  to  have  more  than  an 
element  of  truth  here.  Long  oreshoots,  already  fully  explored  and  mined 
upwards  to  their  terminations  sometimes  have  tongues  projecting  to  the 
daylight;  but  the  average  line  of  the  upper  margin  of  the  long  shoots 
is  perhaps  150  m.  from  the  surface.  Indeed,  there  seems  to  be  a  general 
parallelism  between  the  surface  topography  and  the  configuration  of  the 
oreshoots  below.  This  rule  may  possibly  not  hold  where  there  is  a 
considerable  thickness  of  recent  volcanics,  even  though  such  rocks  are 
cut  by  the  vein  fissures.  It  must  be  admitted  that  there  is  no  rule  with- 
out exceptions  and  that  no  general  statement  is  applicable  to  every  vein 
in  the  district.  There  are  stopes  that  reach  the  surface;  there  are  ore- 
shoots  that  do  not  persist  downwards;  there  are  veins  whose  production 
has  come  from  within  500  ft.  of  the  present  surface  and  others  that  are 
chiefly  barren  to  that  depth  and  productive  below  it.  Nevertheless 
there  is  a  remarkably  persistent  general  relation  between  the  surface  and 
the  upper  limit  of  the  pay  ore. 

This  relation  becomes  still  more  striking  when  we  consider  also  the 
termination  of  the  ore  downwards.  There  seems  to  be  little  doubt  in  the 
minds  of  those  familiar  with  Pachuca  that  the  mineralization  has  a  rather 
abrupt  and  very  definite  termination  in  several  fully  explored  mines,  and 
that  the  productive  area  of  the  veins  is  confined  within  a  vertical  range  of 
about  2000  ft.  (609.6  m.).  Moreover,  in  no  one  oreshoot  is  the  full 
extent  of  this  zone  mineralized.  The  average  height  of  the  pay-ore  zone 
is  perhaps  1500  ft.  (457.2  m.),  in  many  cases  less  and  in  a  few  cases  more 
My  knowledge  of  the  conditions  that  prevail  at  the  bottom  is  largely 
gained  from  others.  It  is  a  matter  of  common  knowledge,  however, 
that  the  veins  scatter,  the  quartz  diminishes,  the  values  fall  off  rapidly 
and  only  occasionally  are  found  any  sulfide  or  base  minerals  such  as 
blende  and  galena.  This  general  situation  is  not  in  any  way  exceptional 
for  veins  in  Tertiary  eruptives,  but  the  explanation  may  not  always  be 
the  same  nor  the  geological  record  so  easily  read. 


HORACE   V.    WINCHELL  7 

Origin    of   Ore 

In  considering  the  possibilities  of  a  mining  district  an  understanding 
of  its  geology  and  something  as  to  the  probable  genesis  of  its  ores  is 
valuable  as  a  guide  in  explorations  and  in  appraising  its  future.  The 
facts  given  may  be  summarized  as  follows: 

1.  The  country  rock  is  a  series  of  Tertiary  volcanics. 

2.  The  vein  fissures  cut  the  entire  series  from  the  oldest  to  the 
youngest. 

3.  Parallel  to  the  veins  are  quartz  porphyry  dikes  of  considerable 
extent.     These  are  not  universally  known,  but  are  in  sufficiently  constant 
association  with  the  veins  to  be  taken  into  consideration  as  possible 
agents  in  ore  genesis. 

4.  The  country  rock  is  not  only  widely  sheared  and  fissured  but 
presents  evidence  of  profound  alteration  by  thermal  waters  over  wide 
zones. 

5.  The  country  rock  is  still  further  altered  by  weathering  from  the 
surface  down  to  considerable  depth. 

6.  The  veins  are  largely  quartz,  but  this  quartz  seldom  comes  to 
the  surface  and  never  in  such  quantity  as  in  the  veins  underground. 
This  statement  is  true  of  veins  and  their  outcrops  whether  found  on  the 
crests  of  mountains  or  in  deep  valleys.     In  other  words,  both  quartz  and 
ore  lie  for  the  most  part  some  distance  below  the  surface. 

7.  Oxidation  and  leaching  persist  to  the  lowest  levels  of  silver  en- 
richment.    Horn  silver,  native  silver,  and  argentite  (the  latter  greatly 
predominating)  are  the  ore  minerals.     They  occur  in  white  quartz,  which 
is  itself  secondary,  or  as  incrustations  or  cavity  fillings  together  with 
pyrite  and  an  occasional  speck  of  chalcopyrite  and  galena.     For  the 
most  part,  oxides  of  iron  and  manganese  are  present  though  in  diminishing 
amount  as  depth  is  gained. 

8.  There  are  seldom  any  massive  sulfide  orebodies  in  the  veins  be- 
neath the  oxidized  ore,  nor  are  there  in  the  upper  levels  the  large  masses 
of  oxidized  material  that  often  indicate  the  former  presence  of  heavy 
sulfide  bodies. 

9.  The  major  axes  of  the  oreshoots  are  more  nearly  horizontal  than 
vertical;  so  much  so  that  the  Pachuca  camp  is  rather  unique  in  this 
respect. 

10.  The  vein  quartz  dies  out  downwards,  and  even  large  veins  dwindle 
into  a  series  of  scattered  and  unmineralized  stringers. 

11.  All  the  mineralized  veins  belong  to  one  general  period  and  contain 
the  same  kind  of  ore.     The  products  of  the  camp  are  silver  and  gold  in 
the  ratio  of  about  5  gm.  of  gold  to  1  kg.  of  silver. 

With  reference  now  to  the  richly  productive  portion  of  the  Pachuca 
camp,  the  history  of  ore  formation  may  well  have  been  somewhat  as 
follows : 


8  GEOLOGY   OF   PACHUCA   AND    EL    ORO,    MEXICO 

Having  accumulated  in  large  mass,  the  andesites,  both  older  and  latter, 
slowly  cooled,  and  zones  of  shearing  were  produced  by  shrinkage  and 
subsidence.  Through  the  multitude  of  cracks  and  fissures  vapors  and 
hot  waters  penetrated  the  rock  and  effected  widespread  alteration,  de- 
positing at  the  same  time  barren  pyrite  in  disseminated  crystals  over 
zones  of  considerable  width.  When  these  fissures  extended  to  sufficient 
depth  there  was  another  outburst  of  magma,  this  time  quartz  porphyry 
and  rhyolite,  followed  by  the  usual  period  of  hot-spring  activity.  At  this 
time  the  first  quartz  was  deposited  in  and  along  the  fissure  already  formed 
and  replacing  and  silicifying  the  broken  andesite  along  the  shear  zones. 
The  period  of  subsidence  and  fracturing  was  not  yet  at  an  end,  for  the 
quartz  porphyry  itself,  after  cooling  sufficiently,  was  fractured  and 
fissured  and  somewhat  mineralized. 

Then,  with  the  dying  down  of  fumarolic  and  hot-spring  activity, 
came  the  opportunity  for  surface  waters,  which  up  to  this  time  had  been 
operating  solely  on  the  exposed  surfaces,  to  begin  working  their  way 
downward  along  the  fissures  and  shear  zones,  oxidizing  and  dissolving 
the  scattered  sulfides  and  carrying  them  to  new  resting  places  at  lower 
levels.  It  is  not  known  how  much  erosion  has  taken  place  since  these 
Tertiary  rocks  were  formed,  but  that  it  may  well  have  been  thousands 
of  feet  is  shown  by  the  depth  of  the  valleys  in  the  immediate  vicinity. 

On  the  western  slope  of  the  mountains  chemical  changes  are  rapid 
and  with  an  already  altered  rock  on  which  to  operate  the  upper  parts  of 
the  veins  were  constantly  and  successively  oxidized,  leached  of  quartz, 
silver  minerals,  pyrite  and  gangue  minerals  while  the  surface  was  eroded 
and  carried  away.  Always  the  values  were  held  in  .the  veins  and  carried 
downward  in  advance  of  the  dissipating  forces  of  erosion,  and  times 
without  number  the  little  films  of  argentite  that  had  been  deposited  at  a 
safe  depth  and  were  becoming  endangered  by  the  slow  approach  of  the 
surface,  were  removed  still  deeper.  In  this  way  we  have  the  cumulative 
result,  first,  of  ages  of  primary  deposition  during  which  perhaps  no 
commercial  orebodies  were  formed,  and,  second,  of  a  long  period  of 
weathering  under  a  hot  sun  and  climatic  conditions  distinctly  favorable 
to  secondary  enrichment. 

The  proof  of  the  theory  is  in  the  character  of  the  minerals,  the  leaching 
and  other  evidences  of  the  work  of  descending  waters,  as  well  as  the 
shape  of  the  oreshoots,  their  correspondence  with  the  topography  of  the 
surface,  the  paucity  of  quartz  at  grass  roots  and  its  diminuendo  habit 
beneath  the  ore  accumulations.  In  short,  all  the  broad  phenomena  of  the 
district  seem  to  be  in  accord  with  this  theory  and  with  no  other.  It  is 
supported  by  both  the  positive  proof  and  the  negative  facts,  by  the  min- 
erals found  as  well  as  by  those  that  are  not  present;  it  is  a  most  excellent 
example  of  the  formation  of  large  and  deep  orebodies  by  secondary  sulfide 
enrichment.  But  there  are  other  features  of  interest. 


HORACE   V.    WINCHELL  9 

Climatic  Effects 

Like  forces  working  on  similar  materials  under  similar  conditions, 
for  an  equal  length  of  time  may  be  expected  to  produce  similar  results. 
Thus,  with  equal  precipitation  and  evaporation  over  the  entire  district, 
uniform  surface  gradients  on  both  sides  of  the  divide  and  similar  rocks 
uniformly  sheared  and  fissured  so  as  to  offer  equal  receptivity  to  drainage 
there  would  probably  result  a  similarity  in  topography  and  in  the  subterra- 
nean products  of  weathering  agencies.  Where  the  materials  are  similar 
but  the  conditions  are  known  to  vary,  it  is  reasonable  to  attribute  heter- 
ogeneity of  products  to  such  variance  of  conditions;  and  where  the  rocks 
are  different,  different  products  may  be  formed  by  the  operation  of 
similar  forces  under  similar  conditions.  At  Pachuca,  the  rocks  are,  in 
general,  similar  mineralogically  and  structurally;  the  operating  forces  are 
similar,  although  not  of  equal  intensity  nor  volume;  the  time  factor  is 
practically  constant;  but  the  products  have  a  wide  variance  indifferent 
portions  of  the  district. 

Reference  has  been  made  to  the  marked  difference  in  the  annual 
precipitation  on  the  western  and  eastern  slopes  of  the  continental  divide, 
but  its  effect  has  not  been  fully  described.  The  first  result  to  be  noticied 
is  in  the  different  sculpturing  of  the  surface.  On  the  eastern  slope  the 
topography  is  rougher,  the  slope  gradients  are  steeper,  the  changes  in 
elevation  more  frequent  and  abrupt.  On  the  western  slope  there  are 
many  long,  smooth,"  gently  sloping  hillsides  and  but  comparatively  few 
sharp  and  deep  canyons.  As  a  consequence,  the  rainfall,  if  equal  over  the 
two  areas,  would  run  off  faster  on  the  eastern  than  on  the  western  slope, 
and  a  smaller  amount  would  percolate  downward  into  the  rocks.  But 
with  the  much  greater  rainfall  on  the  eastern  slope,  it  might  be  expected 
that  the  effect  of  surface  waters  would  be  at  least  as  great,  and  oxidation 
as  deep  in  Real  del  Monte  as  in  Pachuca.  In  fact,  considering  alone  the 
much  greater  precipitation,  one  would  expect  deeper  and  more  intense 
oxidation.  But  the  reverse  is  the  case.  Sulfide  minerals  are  rarely 
found  within  300  ft.  (91  m.)  of  the  surface  in  the  western  part  of  the 
camp,  while  they  are  abundant  within  70  ft.  of  the  surface  about  1  mi. 
(1.6  km.)  east  of  the  Guerrero  mill,  on  the  eastern  side  of  the  range.  It 
is  probable,  as  already  stated,  that  there  is  more  water  in  the  mines  and 
in  the  ground  generally,  on  the  eastern  than  on  the  western  slope.  Why 
then  is  there  such  a  marked  difference  in  the  depth  to  which  oxidation 
has  extended? 

No  doubt  many  factors  enter  into  the  problem.  It  is  evident  at  a 
glance  that  erosion  is  more  rapid,  and  that  it  nearly  keeps  pace  with 
oxidation,  on  the  eastern  slope.  But  there  are  other  reasons  why 
sulfides  in  that  section  are  so  much  nearer  the  surface.  It  may  be  ex- 
plained in  part  by  the  fact  that  the  rainwaters  are  not  so  active  chemically 
after  soaking  through  the  soil.  There  is  perhaps  not  enough  difference 


10  GEOLOGY   OF   PACHUCA   AND    EL    ORO,     MEXICO 

in  temperature  to  make  any  material  difference,  although  such  difference 
probably  exists.  It  is  perhaps  more  largely  due  to  the  fact  that  the  rank 
vegetation  that  covers  the  eastern  foot  hills  deprives  the  rain  of  its 
oxygen  and  hence  the  underground  waters  in  that  section  are  compara- 
tively inert.  In  this  respect  the  Pachuca  camp  offers  a  rare  example  of 
the  effect  of  differing  climatic  conditions  on  the  depth  and  character  of 
mineralization  in  veins. 

In  another  respect,  also,  it  is  interesting.  Many  observers  here  and 
in  other  districts  have  noted  the  fact  that  there  is  oxidation  below  the 
present  water  table  and  have  attributed  the  phenomenon  to  a  change 
of  water  level  in  comparatively  recent  time.  This  does  not  appear  to  be 
the  only,  nor  indeed  always  the  more  probable,  explanation.  Where, 
as  is  probably  the  case  here,  the  entire  volume  of  ground  water  is  slowly 
moving  downward,  and  yet  is  ever  renewed  by  annual  rainfall,  there 
must  be  oxidizing  action  until  all  the  oxygen  is  consumed,  and  thus, 
even  below  the  surface  of  the  subterranean  water  table,  extending  down- 
ward perhaps  several  hundred  feet,  the  sulfide  minerals  will  become 
oxidized.  It  is  only  stagnant  water  or  water  that  has  performed  its 
work  and  become  exhausted,  that  is  inert.  Where  there  is  an  outlet  at 
some  greater  depth  and  the  waters  are  descending,  the  work  of  oxidation 
may  and  frequently  does  proceed  far  beneath  its  apparently  stationary 
level. 

EL  ORO, 'STATE  OF  MEXICO 

For  his  data  on  El  Oro,  the  writer  is  indebted  not  alone  to  his  personal 
examination  but  to  a  report  on  the  camp  by  Dr.  Waldemar  Lindgren, 
written  in  1913.  From  this  report  are  taken  the  following  more  general 
statements,  in  order  to  lay  the  foundation  for  points  that  seem  to  be  of 
special  interest  to  the  economic  geologist: 

"The  district  of  El  Oro  is  situated  on  the  high  plateau  of  Mexico, 
near  its  western  edge,  at  an  elevation  of  about  10,000  ft.  (3048  m.). 
On  this  part  of  the  plateau  broad  valleys  are  separated  by  irregular 
groups  of  mountains  rising  2000  to  3000  ft.  above  the  depressions.  The 
valleys  are  filled  with  volcanic  tuff  and  detritus.  The  mountains  are 
largely  built  up  of  volcanic  flows,  mainly  andesite,  but  at  many  places 
the  underlying  older  rocks  are  exposed.  The  latter  consist  of  calcareous 
shales  of  Cretaceous  or  Jurassic  age  and  in  places  contain  an  older  series 
of  igneous  rocks  intruded  into  the  shales  and  exposed  by  erosion. 

'  *  The  geological  sequence  is  then  as  follows : 

1.  Calcareous  shale  with  some  sandstone  and  limestone. 

2.  Older  Tertiary  igneous  rocks  intruded  into  or  poured  out  on  these 
sedimentary  shales. 

3.  Formation  of  fissure  veins  intersecting  shales  and  older  igneous 
rocks. 


HORACE  V.    WINCHELL  11 

4.  Epoch  of  erosion. 

5.  Late  Tertiary  and  recent  igneous  rocks,  chiefly  flows  of  lava  and 
tuffs,  resting  on  eroded  shales,  older  igneous  rocks  and  veins  and  showing 
no  mineralization. 

6.  Recent  epoch  of  erosion. 

The  Formations 

"Later  Andesite. — The  younger  surface  lavas  are  mostly  massive, 
dark  gray  hornblende-andesites  which  are  oxidized  and  disintegrated  near 
the  surface,  but  show  no  mineralization,  nor  do  they  contain  pyrite. 
Toward  the  valley  agglomerates  and  tuffs  gradually  take  the  place  of  the 
massive  rocks.  Where  the  contact  with  the  shales  is  exposed  by  mining 
operations,  a  few  feet  of  reddish  stratified  material  of  fragmental  origin 
often  rest  directly  on  the  shale. 

"  The  thickness  of  this  lava  is  manifestly  affected  by  the  recent  erosion. 
Along  the  San  Rafael  lode,  south  of  North  shaft,  it  is  less  than  200  ft. 
(60.9  m.),  but  north  of  this  point  it  increases  to  400  ft.  (121.9  m.)  and 
at  Tiro  Hondo  and  San  Patricio  shafts  it  is  about  600  ft.  Under  the 
summit  of  the  hill  the  thickness  is  1000  ft.  Dikes  and  intrusive  necks  of- 
this  lava  are  found  in  the  adjoining  Esperanza  mine. 

"  The  Older  Andesite. — The  older  andesite  is  a  greenish  dense  rock 
which  has  been  greatly  altered  by  the  vein-forming  agencies  and  now 
contains  much  pyrite,  calcite  and  sericite.  It  occurs  as  thick  intrusive 
sheets,  or  as  irregular  masses  in  the  black  shales,  also  as  smaller  dikes. 
On  the  property  of  the  El  Oro  Co.  none  of  this  rock  reaches  the  present 
surface  or  the  old  surface  underneath  the  younger  andesite,  but  it  forms  a 
thick  flat  body  which  is  about  600  ft.  thick;  it  was  first  encountered  in 
the  northern  part  of  the  property  along  the  San  Rafael  vein  about  600 
ft.  below  the  capping.  Farther  south  it  lies  deeper,  being  near  the 
Interior  shaft  at  about  900  ft.  below  the  capping;  and  in  the  Carmen  mine 
its  top  lies  again  about  800  ft.  below  that  surface.  The  lower  contact 
with  the  shales  has  been  found  in  the  northern  part  of  the  mine.  Through- 
out, this  andesite  sheet,  or  sill,  is  faulted  by  the  fissure  of  the  San  Rafael, 
the  vertical  throw  being,  in  the  vicinity  of  the  Somera  shaft,  about  670 
feet. 

"Down  to  about  the  1300-ft.  level,  the  andesite  appears  only  in  the 
foot  wall.  Below  the  1300-ft.  level,  it  begins  to  appear  in  the  hanging 
wall,  and  continues  to  form  that  wall  down  to  the  1600-ft.  level,  the 
lowest  point  reached."  (Since  Doctor  Lindgren's  examination  the  mine 
workings  have  developed  ground  far  beneath  the  lower  margin  of  this 
sill). 

"The  older  series  of  andesite  is  represented  both  by  intrusive  rocks 
and  lava  flows. 

11  The  Sedimentary  Rocks. — The  predominating  rock  is  a  black,  bitu- 


12  GEOLOGY   OF   PACHUCA   AND   EL   ORO,    MEXICO 

minous  shale,  well  stratified  and  often  containing  much  calcite,  in  fact 
grading  into  a  calcareous  shale  and  occasionally  into  a  black  granular 
limestone.  In  places  the  shale  contains  embedded  masses  of  a  dark  gray 
friable  sandstone.  This  sandstone  is  more  abundant  in  the  deep  levels, 
and  is  typically  present  at  the  station  and  crosscut  of  the  Somera  shaft 
on  the  1300-ft.  level.  When  examined  in  the  field  this  rock  was  held  to 
be  of  tuffaceous  origin,  but  the  microscope  has  shown  it  to  be  a  pure 
quartzose  sandstone. 

"The  sedimentary  rocks  lie  horizontal,  or  at  slight  dips  that  exhibit 
no  marked  irregularity.  A  total  thickness  of  1300  ft.  of  strata  is  exposed 
in  the  workings." 

The  Veins 

We  have  at  El  Oro  a  series  of  veins  bearing  gold  and  silver  minerals 
in  a  quartz-calcite  gangue.  These  veins  are  fissures  that  were  filled  by 
replacement  and  infiltration.  Some  of  them  are  evidently  fissures  of 
considerable  displacement.  They  cut  through  the  black  shales  and 
through  the  sill  of  andesite  that  was  intruded  horizontally  into  these 
shales,  and  which,  as  a  natural  consequence,  is  both  overlain  and  under- 
lain by  the  shale  beds.  After  the  veins  were  formed  and  mineralized, 
they  were  subjected  to  the  action  of  surface  waters  for  a  considerable 
time,  and  an  unknown  extent  of  their  upper  portions  was  removed  by 
erosion.  They  are  oxidized  to  the  depth  of  nearly  1000  ft.,  and  their 
silver  content  at  least  was  secondarily  enriched  by  the  action  of  descending 
waters.  The  general  strike  of  the  veins  is  about  north  30°  west,  and 
their  dip  at  varying  angles  to  the  west.  They  have  branches  in  both 
foot  and  hanging  walls,  but  more  numerously  in  the  latter. 

After  a  long  period  of  weathering  and  erosion  the  country  was  covered 
by  more  recent  lava  flows,  which  were  in  turn  weathered  and  eroded  until 
in  some  localities  the  underlying  shales  and  one  vein,  the  first  discovered 
Descubridora,  are  exposed  on  the  surface.  These  lava  flows  and  volcanic 
tuffs  are  not  penetrated  by  the  veins  which  cut  the  underlying  rocks. 

The  veins  are,  in  turn,  cut  and  displaced  by  north-dipping  east-west 
faults.  These  faults  are  unmineralized,  except  by  a  little  calcite,  which 
may  be  of  recent  deposition.  Their  general  effect  has  been  to  step  the 
country  down  to  the  north.  The  direction  of  movement  along  several  of 
these  faults  has  been  diagonally  downwards  to  the  east. 

The  andesite  sill  varies  in  thickness  from  400  to  possibly  more  than 
700  ft. ;  it  has  been  cut  and  displaced  by  the  vein  fissures  with  throws  of 
several  hundred  feet. 

The  most  productive  orebodies  have  been  found  in  the  veins  where 
they  lie  within  the  shale-  overlying  the  andesite  sill  and  adjacent  to  it 
where,  by  reason  of  faulting  movement  along  the  vein  fissures,  the  shale 
is  brought  in  opposition  to  the  andesite.  There  is  also  some  ore  found 


HORACE  V.    WINCHELL 


13 


where  the  veins  lie  wholly  in  andesite,  but  not  in  large  quantity  anywhere 
above  the  lower  margin  of  the  lower  faulted  segment  of  the  sill.  Develop- 
ments indicate  that  the  veins  are  not  enriched  for  any  distance  below  the 
andesite.  Quartz  indeed  is  found  to  persist  for  some  distance  beneath 
it,  but  with  diminishing  tendency  downward,  and  with  smaller  oreshoots. 


'Later  Andes/fe  • 


1 .  *  • '  "O/c/er  Andesite 


Q/der.^ 
ndesite,, 


^>ShJfe  and  SaZc/sto 


APPROXIMATE  CROSS  SECTION 

AT 


ELORO  MEXICO 


LOOKING   NORTHERLY 


FIG.  l. 

The  quartz  scatters  in  stringers  of  diminishing  strength  and  dies  out. 
The-  lower  stretches  of  the  veins  contain  some  of  the  baser  sulfides,  such  as 
blende  and  galena. 

The  commercial  orebodies  occur  in  long  relatively  horizontal  ore- 
shoots  better  mineralized  in  the  upper  levels.  Some  of  these  shoots  have 
been  definitely  bottomed  while  others  are  still  being  pursued  through 
their  downward  reaching  lobes  and  tails. 


14  GEOLOGY   OF   PACHUCA   AND   EL   ORO,   MEXICO 

Origin  of  the  Ore 

It  is  seldom  that  the  facts  observable  suffice  to  point  strongly  to  a 
particular  mass  of  eruptive  rock  as  the  source  of  vein  mineralization. 
In  this  respect  El  Oro  is  of  particular  interest.  The  position  of  the  ore- 
bodies  in  the  ground  and  their  relation  to  the  andesite  sill,  the  dwindling 
of  the  quartz  and  mineralization  downward  suggest  that  it  was  the 
fountain  head  of  ore  deposition.  There  is  nothing  to  suggest  the  presence 
of  other  sills  or  masses  of  intrusive  rock  and  many  facts  that  lead  one  to 
doubt  their  existence.  This  holds  true  of  all  the  veins  thus  far  developed. 
They  are  large  and  rich  above  the  sill  and  poor  and  small  or  entirely 
pinched  out  beneath  it.  This  could  hardly  be  the  case  if  they  had  been 
formed  from  solutions  rising  from  greater  depth.  Further  careful  study 
should  prove  or  disprove  the  integrity  of  this  theory.  It  is  at  least 
useful  as  a  working  hypothesis  for  it  not  only  suggests  the  futility  of 
costly  exploration  beneath  the  sill  but  at  once  shows  the  attractiveness  of 
territory  overlying  the  sill  still  unexplored  and  now  covered  by  later 
andesite,  which  caps  the  vein-carrying  shale  formation. 

Value  of  Geological  Study 

In  these  two  camps,  we  thus  find  geological  data  that  encourage 
further  exploration  and  indicate  where  it  is  most  likely  to  be  successful, 
while  in  many  camps  are  found  facts  that,  properly  interpreted  demon- 
strate the  utter  folly  of  large  expenditures  in  the  search  for  orebodies, 
the  existence  of  which  is  extremely  problematical.  Such  explorations 
are  now  in  progress  in  some  of  the  older  western  mining  camps  and 
hundreds  of  thousands  of  dollars  are  being  expended  in  utter  defiance  of 
the  easily  read  geologica-1  history  of  the  districts  wherein  such  work  is 
being  prosecuted. 


